Solar cell, method for manufacturing same and solar cell module

ABSTRACT

The solar cell includes a plurality of light-receiving-side finger electrodes on a light-receiving surface of a photoelectric conversion section having a semiconductor junction. The light-receiving surface of the photoelectric conversion section is covered with a first insulating layer. Each light-receiving-side finger electrodes include: a first metal seed layer provided between the photoelectric conversion section and the first insulating layer; and a first plating metal layer being conduction with the first metal seed layer through openings formed in the first insulating layer. The solar cell includes an isolated plating metal layer pieces contacting neither the light-receiving-side finger electrodes nor the back-side finger electrodes. On the surface of the first insulating layer, an isolated plating metal crowded region is present in a form of a band-shape extending parallel to an extending direction of the light-receiving-side finger electrodes.

CROSS REFERENCE TO RELATED APPLICATION

This present application is a national stage filing under 35 U.S.C. §371 of PCT application number PCT/JP2016/050406 filed on Jan. 7, 2016which is based upon and claims the benefit of priority to JapanesePatent Application No. 2015-001329 filed on Jan. 7, 2015 in the JPO(Japanese Patent Office). The disclosures of the above-listedapplications are hereby incorporated by reference herein in theirentirety.

TECHNICAL FIELD

The invention relates to solar cell, manufacturing method thereof andsolar cell module.

BACKGROUND ART

In a solar cell, carriers (electrons and holes) generated by lightirradiation to a photoelectric conversion section composed of asemiconductor junction are extracted to an external circuit to generateelectricity. Metal electrodes are provided on front surface and backsurface of the photoelectric conversion section of the solar cell forefficiently extracting carriers generated at the photoelectricconversion section to the external circuit. For example, in aheterojunction solar cell in which a silicon-based thin-film is providedon a surface of a conductive single-crystalline silicon substrate toform a semiconductor junction, transparent electrode made of atransparent electroconductive oxide or the like is provided on asilicon-based thin-films on each of a light-receiving side and a backside, and metal collecting electrodes are provided on the transparentelectrodes to collect photocarriers generated in the crystalline siliconsubstrate.

Light that is irradiated on regions where the metal collectingelectrodes are formed is reflected on or absorbed by the metalcollecting electrodes to cause shadowing loss. In order to reduce theshadowing loss, the collecting electrode on the light-receiving side isformed in a pattern shape. A typical example of the pattern of thecollecting electrode is a grid pattern composed of finger electrodes andbus bar electrodes. The collecting electrode on the back side may bedisposed on the entire surface of the back surface, or may be a patternshape. In a solar cell module installed in such a manner that lightenters into the module from back side as well as from front side, e.g.,a flat roof type and on-ground installation type solar cell module, apatterned collecting electrode is provided on the back side of the solarcell. Also in a solar cell module having a structure in which lightincident to an interspace between adjacent two cells is reflected on aback sheet, a patterned collecting electrode is provided on the backside of the solar cell.

The patterned collecting electrode is generally formed by screenprinting of an electroconductive paste such as silver paste. Thecollecting electrode formed by using silver paste contains a resinmaterial, so that it is high in resistivity and high in material costs.Methods for forming metal collecting electrodes by plating are proposedfor reduction of the electrode material costs and the like. The platingmethod makes it possible to form a metal electrode large in thicknessand low in resistance, so that the line width of the metal electrode ismade smaller than the method using the electroconductive paste.Accordingly, formation of the metal collecting electrode by the platingmethod also has an advantage of improving the light collectionefficiency of the resultant solar cell by decreasing a shadowing loss.

A method of forming a collecting electrode having a predeterminedpattern by plating is known. In such a method, an insulating layerhaving openings is formed on a surface of a photoelectric conversionsection and a metal is deposited on the surface of the photoelectricconversion section in areas where the openings are formed in theinsulating layer. For example, Patent Document 1 discloses a method offorming an insulating layer having a thickness of about 10 to 15 μm on atransparent electrode of a photoelectric conversion section, makingopenings in the insulating layer, and then forming a collectingelectrode by electroplating.

Patent Documents 2 and 3 propose a method of forming crack-like openingsin an insulating layer deposited on the metal seed layer. In thismethod, a metal seed layer is formed by printing an electroconductivepaste containing a low-melting-point material, and an insulating layeris formed thereon, followed by heating for annealing so that thelow-melting-point material in the metal seed layer is thermallyfluidized. This method is excellent from the viewpoint of material costsand process costs, since the method makes it possible to form openingsselectively in the metal seed layer-formed region of the insulatinglayer, and thus any patterning of the insulating layer, such as using aresist and the like, is unnecessary.

PRIOR ART DOCUMENT Patent Documents

Patent Document 1: International Publication No. WO 2012/029847

Patent Document 2: International Publication No. WO 2013/077038

Patent Document 3: International Publication No. WO 2014/185537

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As described above, by forming a metal collecting electrode by a platingmethod, the resultant solar cell may have decreased electrode formationarea thereby reducing shadowing loss. However, when the electrodeinterval between adjacent two finger electrodes is widened to decreasethe electrode formation area, the carrier collecting efficiency of thesolar cell tends to decline, and a fill factor of the solar cell tendsto be lowered. Thus, the pattern shape of the electrodes is determinedby considering the balance between the shadowing loss and the carriercollecting efficiency. Regarding a collecting electrode on thelight-receiving side, the interval between adjacent two fingerelectrodes is set so that the total area of the finger electrodes iswithin the range of about 1 to 3% of the whole of the light-receivingsurface area. In order to further improve the conversion efficiency ofthe solar cell, it is desired that while the electrode formation area ismaintained, the shadowing loss is decreased to improve light-capturingefficiency.

Means for Solving the Problems

The inventors have found that by depositing a plating metal having asmall area on a collecting electrode-non-formed region on alight-receiving side of a solar cell, the light capturing efficiency isimproved despite an increase in light shielding area by the metal.

The solar cell of the present invention includes a photoelectricconversion section having a semiconductor junction, a plurality oflight-receiving-side finger electrodes provided on a light-receivingsurface of the photoelectric conversion section, and a plurality ofback-side finger electrodes provided on a back surface of thephotoelectric conversion section. It is preferred that a separationdistance between the adjacent two light-receiving-side finger electrodesis larger than a separation distance between the adjacent two back-sidefinger electrodes.

The light-receiving surface of the photoelectric conversion section iscovered with a first insulating layer. The light-receiving-side fingerelectrodes each include a first metal seed layer and a first platingmetal layer. The first metal seed layer is provided between thephotoelectric conversion section and the first insulating layer. Thefirst plating metal layer is in conduction with the first metal seedlayer through openings formed in the first insulating layer.

The solar cell of the present invention has isolated plating metal layerpieces that contact neither the light-receiving-side finger electrodesnor the back-side finger electrodes. On a surface of the firstinsulating layer, an isolated plating metal crowded region is present ina form of a band-shape extending parallel to an extending direction ofthe light-receiving-side finger electrodes. An area density of theisolated plating metal layer pieces in the isolated plating metalcrowded region is two or more times an average area density in a wholeof a region where the light-receiving-side finger electrodes are notformed. It is preferred the isolated plating metal crowded region islocated 20 μm or more apart from the light-receiving-side fingerelectrodes.

It is preferred that the back surface of the photoelectric conversionsection is covered with a second insulating layer, and that theback-side finger electrodes each include a second metal seed layer and asecond plating metal layer. The second metal seed layer is providedbetween the photoelectric conversion section and the second insulatinglayer. The second metal seed layer is in conduction with the secondplating metal layer through openings formed in the second insulatinglayer. In this configuration, it is preferred that the isolated platingmetal layer pieces deposited on a surface of the first insulating layerare larger in area density than the isolated plating metal layer piecesdeposited on a surface of the second insulating layer.

Each of the first insulating layer and the second insulating layer ispreferably an inorganic layer. The first insulating layer and the secondinsulating layer each preferably have a thickness of 10 to 200 nm. Thefirst plating metal layer, the second plating metal layer and theisolated plating metal layer pieces each preferably contain copper.

The finger electrodes of the solar cell of the present invention can beproduced by forming plating metal layers on a metal seed by a platingmethod through openings formed in the respective insulating layersdeposited on the metal seed. When an electroconductive paste is printedto form a metal seed layer, solvent may ooze out from anelectroconductive paste printed region. In this case, isolated platingmetal layer pieces are easily formed on the insulating layer near theouter edge of a region where the solvent has oozed. Therefore, theisolated plating metal crowded region is formed in a band-shapeextending parallel to the finger-electrode extending direction.

The present invention further relates to a solar cell module includingsolar cells as described above. The solar cell module of the presentinvention includes a light-receiving-side protecting member disposed onthe light-receiving side of the solar cells, and a back-side protectingmember disposed on the back side of the solar cell. Encapsulants areprovided between the solar cell and the light-receiving-side protectingmember and between the solar cell and the back-side protecting member.

The light-receiving-side protecting member is transparent, and ispreferably made of glass. The back-side protecting member may be eithertransparent or opaque. The back-side protecting member preferablyincludes no metal foil. The encapsulant provided between the solar celland the back-side protecting member preferably includes a polyolefinresin.

Effects of the Invention

In the solar cell of the present invention, light reflected on thefinger electrodes on the light-receiving side can be scattered on theisolated plating metal layer pieces to enter into the photoelectricconversion section. Thus, the solar cell is excellent in lightcollection efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a solar cell of an embodiment.

FIG. 2 is a plan view of a light-receiving surface of a solar cell.

FIG. 3 is a plan view of a back surface of the solar cell.

FIG. 4 is a schematic sectional view of a solar cell module of oneembodiment.

FIG. 5 is a microscope-observed photograph of a light-receiving surfaceof a solar cell of an example.

FIG. 6 is a microscope-observed photograph of a back surface of thesolar cell of the example.

MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a schematic sectional view of a heterojunction solar cellaccording to an embodiment of the present invention. As schematicallyillustrated in FIG. 1, the solar cell of the invention includes, on alight-receiving side of a photoelectric conversion section 50, aplurality of light-receiving-side finger electrodes 60, and includes, ona back side of the photoelectric conversion section 50, a plurality ofback-side finger electrodes 70. The light-receiving-side fingerelectrodes 60 each include a metal seed layer 61 and a plating metallayer 62 in this order from the photoelectric conversion section 50.

Hereinafter, the present invention will be described more detail byshowing an example of the above-mentioned heterojunction solar cell. Aheterojunction solar cell is a crystalline silicon solar cell in which asemiconductor junction is produced by providing a silicon-basedthin-film on a surface of a conductive single-crystalline siliconsubstrate, the silicon-based thin-film having a bandgap different fromthat of the single-crystalline silicon. It is known that surface defectsof the conductive single-crystalline silicon substrate 10 are terminatedand thus conversion efficiency is improved, when intrinsic silicon-basedthin-films 21 and 22 are interposed between conductive silicon-basedthin-films 31 and 32 for producing diffusion potential and a conductivesingle-crystalline silicon substrate 10, respectively.

[Photoelectric Conversion Section Structure]

(Silicon Substrate)

The conductive single-crystalline silicon substrate 10 may be an n-typesingle-crystalline silicon substrate or a p-type single-crystallinesilicon substrate. An n-type single-crystalline silicon substrate ispreferred because of a long carrier life time in the silicon substrate.

(Silicon-Based Thin-Film)

The silicon-based thin-film 31 of first conductivity-type is formed on afirst principal surface (on the light-receiving side) of the conductivesingle-crystalline silicon substrate 10, and the silicon-based thin-filmof second conductivity-type is formed on a second principal surface (onthe back side) of the conductive single-crystalline silicon substrate10. The silicon-based thin-film 31 of first conductivity-type and thesilicon-based thin-film 32 of second conductivity-type have differentconductivity-types. One of these conductivity-types is a p-type whilethe other is an n-type. These conductive silicon-based thin-films 31 and32 may be a p-type silicon-based thin-film and an n-type silicon-basedthin-film. The thickness of each of the conductive silicon-basedthin-films is preferably 20 nm or less, more preferably 15 nm or less.The thickness of the conductive silicon-based thin-film is preferably to2 nm or more to keep appropriate film coverage.

Out of the conductive silicon-based thin-films 31 and 32, a siliconlayer different in conductivity-type from the conductivesingle-crystalline silicon substrate 10 is called an “emitter layer”.The following structure is called a “front emitter structure” in whichthe conductive single-crystalline silicon substrate 10 and thesilicon-based thin-film 31 of first conductivity-type formed on thelight-receiving side have opposite conductivity-types, and theconductive single-crystalline silicon substrate 10 and the silicon-basedthin-film 32 of second conductivity-type formed on the back side of thesolar cell have the same conductivity-type. For example, in aheterojunction solar cell having a “front emitter structure” in whichthe conductive single-crystalline silicon substrate 10 is an n-typesingle-crystalline silicon substrate, a p-type silicon-based thin-filmwhich becomes an emitter layer is arranged on the light-receiving side,and an n-type silicon-based thin-film is arranged on the back side. Thefollowing structure is called a “back emitter structure”: a structure inwhich the conductive single-crystalline silicon substrate 10 and thesilicon-based thin-film 31 of first conductivity-type have the sameconductivity-type, and the conductive single-crystalline siliconsubstrate 10 and the silicon-based thin-film 32 of secondconductivity-type have opposite conductivity types.

Each of the conductive silicon-based thin-films 31 and 32 is preferablymade of amorphous silicon. A dopant impurity is preferably P(phosphorus) for an n-type silicon layer, and is preferably B (boron)for the p-type silicon layer.

It is preferred that the intrinsic silicon-based thin-film 21 isprovided between the silicon substrate 10 and the silicon-basedthin-film 31 of first conductivity-type, and the intrinsic silicon-basedthin-film 22 is provided between the silicon substrate 10 and thesilicon-based thin-film 32 of second conductivity-type. When theintrinsic silicon-based thin-films 21 and 22 are formed on therespective surfaces of the conductive single-crystalline siliconsubstrate 10, surface defects of the silicon substrate 10 are terminatedso that the lifetime of carriers is elongated to improve output power ofthe solar cell.

The method for forming each of the silicon-based thin-films is notparticularly limited, and is preferably a CVD (chemical vapordeposition) method since the method can attain a precise thicknesscontrol. A raw-material gas used for the CVD may be a silicon-based gassuch as SiH₄, or may be a mixed gas of a silicon-containing gas and H₂.In order to improve the silicon-based thin-film in optical transparencyand the like, a small amount of raw material gas containing, oxygen,carbon or the like may be added to the silicon-based gas. A dopant gasused to form the conductive silicon-based thin-film is, for example,B₂H₆ or PH₃.

(Transparent Electrode Layer)

The photoelectric conversion section 50 of the heterojunction solar cellincludes transparent electrode layers 41 and 42 on the conductivesilicon-based thin-films 31 and 32, respectively. The raw material ofthe transparent electrode layers 41 and 42 may be generally atransparent conductive metal oxide such as indium oxide, tin oxide, zincoxide, titanium oxide, or a complex oxide of two or more of such oxides.In particular, an indium-based complex oxide made mainly of indium oxideis preferred from the viewpoint of achieving both highelectroconductivity and transparency. A dope impurity for theindium-based complex oxide may be a metal such as Sn, Ti, W, Ce or Ga,or a metal oxide of these metals.

The thickness of each of the transparent electrode layers 41 and 42 arepreferably from 40 to 80 nm, more preferably from 50 to 70 nm. Themethod for forming the transparent electrode layer is not particularlylimited, and is preferably a sputtering method or an RPD (radical plasmadeposition) method since the method can attain a precise thicknesscontrol.

When a transparent electrode is formed in an in-plane edge portion onthe emitter layer side (e.g., the side where the silicon layer differentin conductivity-type from that of the silicon substrate is located),recombination or leakage is easily caused. It is therefore preferredthat an end portion and a peripheral portion of the substrate is coveredwith a mask during formation of the transparent electrode layer on theemitter layer side. The “peripheral portion” means a region of thesubstrate that extends over a length of about 300 μm to 1000 μm from theend portion of the substrate. In the meantime, since thenon-emitter-layer side (the side where the silicon layer having sameconductivity-type to the silicon substrate is arranged) is relativelyless affected by the recombination and the leakage, the outputted poweris not easily lowered even when the transparent electrode layer isformed on the entire surface on the non-emitter-layer side.

[Collecting Electrode]

A light-receiving-side collecting electrode is formed on the firsttransparent electrode layer 41 on the light-receiving side, and aback-side collecting electrode is formed on the second transparentelectrode layer 42 on the back side. Since the solar cell of the presentinvention has patterned collecting electrodes on both thelight-receiving side and the back side, light is captured from the backside as well.

The light-receiving-side collecting electrode and the back-sidecollecting electrode each include a plurality of finger electrodes.Respective directions along which the finger electrodes are extended arepreferably parallel to each other. FIG. 2 is a plan view of thelight-receiving side of the solar cell, and FIG. 3 is a plan view of theback side of the solar cell. A circled portion on the right side of eachof FIGS. 2 and 3 is an enlarged view of a vicinity of the fingerelectrodes.

In the configuration illustrated in FIG. 2, finger electrodes 60 and busbar electrodes 66 extending orthogonal to the finger electrodes areprovided on the light-receiving surface of the photoelectric conversionsection. In the configuration illustrated in FIG. 3, finger electrodes70 and bus bar electrodes 76 extending orthogonal to the fingerelectrodes are provided on the back surface of the photoelectricconversion section. Carrier collecting efficiency can be improved whenbus bar electrodes orthogonal to the finger electrode are provided toform grid shape collecting electrodes as illustrated in each of FIGS. 2and 3. In addition, electrical connection between the solar cell andwiring members can be facilitated by providing the bus bar electrodes.

Since the amount of light incident from the back side is 10% or less ofthat from the light-receiving side, shadowing loss caused by the fingerelectrodes on the back side is less than those on the light-receivingside. It is therefore preferred to design the back-side fingerelectrodes, prioritizing an improvement of carrier collectingefficiency. It is preferred that the back-side finger electrodes areprovided more densely than the light-receiving-side finger electrodes,and a separation distance between the light-receiving-side fingerelectrodes is greater than that of the back-side finger electrodes. Thefinger electrode separation distance is a distance between respectivecenter lines of the adjacent two finger electrodes. The separationdistance of the light-receiving-side finger electrodes is preferablyfrom 1.5 to 5 times, more preferably from 2 to 4 times greater than thatof the back-side finger electrodes.

As illustrated in FIG. 1, the light-receiving-side finger electrode 60has a plating metal layer 62 on the metal seed layer 61. The metal seedlayer 61 is formed between the photoelectric conversion section 50 andan insulating layer 81. The plating metal layer 62 is in conduction withthe metal seed layer 61 through the openings 86 formed in the insulatinglayer 81.

(Metal Seed Layer)

The metal seed layer 61 functions as an underlying layer of the platingmetal layer 62. Examples of the metal contained in the metal seed layerinclude Au, Ag, Ni, Cu, Sn and Al. Among these, Ag, Ni, and Sn arepreferred to keep the contact resistance low between the metal seedlayer and the surface of the photoelectric conversion section, and tosuppress resistivity increase and the like caused by oxidization. Acombination of a plurality of metallic materials may be used in order tomaintain the reliability of the metal seed layer and simultaneouslydecrease costs.

The metal seed layer 61 may be formed by, for example, electrolessplating, sputtering, vapor deposition, or printing. From the viewpointof raw material utilization efficiency, the metal seed layer ispreferably formed by printing. In formation of the metal seed layer byprinting, an electroconductive paste containing metal fine particles, abinder resin material and a solvent is preferably used. The binder resinis preferably a thermosetting resin such as epoxy resin, phenolic resin,or acrylic resin. The resin may be a solid resin or a liquid resin.

(Plating Metal Layer)

The plating metal layer 62 is formed on the metal seed layer 61. In thiscase, collecting electrode with lower resistance can be formed at lowercosts than formation of collecting electrode using only Ag paste.Examples of the metal deposited for the plating metal layer include Sn,Cu, Ag and Ni. Among these, Cu is preferred due to even lower resistanceat even lower costs.

Although the plating metal layer 62 may be formed by either electrolessplating or electroplating, electroplating is preferred. Electroplatingis favorable in view of productivity, since the electroplating can givea large metal deposition rate, and metal deposition thickness can becontrolled based on a coulomb amount.

The plating metal layer 62 may be composed of a plurality of layers. Forexample, formation of a plating metal layer with highelectroconductivity, such as Cu, followed by formation of a platingmetal layer with excellent chemical stability, such as Sn, may suppressdeterioration of the plating layer caused by oxidation etc.

The structure of the back-side finger electrodes 70 is not particularlylimited, and is preferably a structure equivalent to that of thelight-receiving-side finger electrodes 60, i.e., a plating metal layer72 is provided on a metal seed layer 71, and the metal seed layer 71 isin conduction with the plating metal layer 72 through openings 87 formedin an insulating layer 82.

The raw materials of the first plating metal layer 62 of thelight-receiving-side finger electrodes 60 and the second plating metallayer 72 of the back-side finger electrodes 70 may be same or differentfrom each other. When the respective raw materials of the front side andback side plating metal layers are identical with each other, stressesand thermal expansions on front and back side of the photoelectricconversion section can be made even. In addition, the first platingmetal layer 62 on the light-receiving surface and the second platingmetal layer 72 on the back surface can be formed simultaneously in asingle plating bath, so that the process can be made simple to improvethe production efficiency while lowering process costs.

The method for forming the bus bar electrodes 66 and 76 is notparticularly limited and is preferably equivalent to the method forforming the finger electrodes 60 and 70, in which a plating metal layeris formed on a metal seed layer. By forming the finger electrodes andthe bus bar electrodes through the same method, the process can be madesimple to improve the productivity. In addition, since the bus barelectrodes are larger in line width than the finger electrodes, platingmetal layers with high in-plane uniformity can be formed by anelectroplating with providing electric supply points on the metal seedlayers of the bus bar electrodes.

[Insulating Layer]

In the collecting electrode-non-formed region on the light-receivingsurface of the photoelectric conversion section, a substantially entiresurface is covered with the insulating layer 81. The wording“substantially entire surface” means an area region of 95% or more ofthe surface. In order to enhance a water-vapor-barrier effect or ahydrogen-elimination-preventing effect of the insulating layer, coverageof the collecting electrode-non-formed region with the insulating layeris preferably 98% or more, more preferably 99% or more. It is alsopreferred that the substantially entire collecting electrode non-formedregion on the back surface of the photoelectric conversion section iscovered with the insulating layer 82.

The formation of the first insulating layer 81 of the light-receivingsurface and the second insulating layer 82 of the back surface makes itpossible to protect the photoelectric conversion section 50 from theplating solution during formation of the first and second plating metallayers 62 and 72. By making the opening 86 and 87 in the insulatinglayers 81 and 82 on the metal seed layers 61 and 71, the plating metallayers 62 and 72 can be selectively formed on the metal seed layers 61and 71, respectively. In the present invention, isolated plating metallayer pieces 69 can be formed at specific positions of thelight-receiving surface by providing openings 89 in metal seed layer61-non-formed regions of the first insulating layer 81.

Materials of the insulating layers 81 and 82 are not particularlylimited. An inorganic material is preferred since openings can easily beformed and the material may have excellent protecting performance. Theinorganic material of the insulating layers may be a metal oxide such assilicon oxide, magnesium oxide, copper oxide, or niobium oxide. Theinorganic material of the insulating layers is preferably, for example,SiO, SiN or SiON since the material is easily formed by CVD or printing,and is excellent in transparency. In order to improve light-collectionefficiency of the solar cell module, the respective refractive indexesof the insulating layers 81 and 82 are preferably lower than therespective refractive indexes of the outermost layers 41 and 42 of thephotoelectric conversion section 50 and higher than the respectiverefractive indexes of encapsulants 111 and 112.

The thickness of each of the insulating layers 81 and 82 is notparticularly limited. In order for these layers to achieve bothprotectability for the photoelectric conversion section 50 and easinessfor making the openings in the layers, the thickness of each of thefirst insulating layer 81 and the second insulating layer 82 ispreferably from 10 to 200 nm, more preferably from 30 to 150 nm.

The insulating layers each made of an inorganic material are high inwater vapor barrier performance, so that the layers also have an effectof protecting the transparent electrode layers 41 and 42 formed on therespective surfaces of the photoelectric conversion section 50 fromwater in the environment. The respective surfaces of the photoelectricconversion section are covered with the inorganic insulating layers 81and 82, and this can give a solar cell module high in reliability evenwhen a back-side protecting member 130 contains no metal foil.Accordingly, the back-side protecting member 130 containing no metalfoil is usable, so that it is possible to improve the conversionefficiency of the solar cell module of an installation type, in whichlight enters into the module also from the back side thereof such as aflat roof type or an on-ground installation type.

The method for forming the insulating layers is not particularlylimited. When the metal seed layers 61 and 71 are formed by printing anelectroconductive paste, the insulating layers 81 and 82 are formedpreferably by CVD. As described in WO 2013/077038 (Patent Document 1)above, according to a method of forming a metal seed layer by the screenprinting of, e.g., an electroconductive paste, and then forming aninorganic insulating layer made of, e.g., silicon oxide thereon by CVD,the surface shape of the metal seed layer can be changed by heatingduring or after the film formation by the CVD, so that crack-likeopenings can be formed in an insulating layer on the metal seed layer.The metal seed layers 61 and 71 exposed to the atmosphere at the bottomsof the openings 86 and 87 in the respective insulating layers 81 and 82become origination points for plating, so that the plating metal layers62 and 72 can be selectively formed on the metal seed layer-formedregions.

[Isolated Plating Metal Layer Piece]

The solar cell of the invention has isolated plating metal layer pieces69 which do not contact any finger electrode on the surface of the firstinsulating layer 81 of the light-receiving surface. The isolated platingmetal layer pieces are not in conduction with any finger electrode, andthus do not contribute to extracting photocarriers in the solar cell.Light near the surface of the solar cell is reflected and scattered tochange propagation direction largely due to the isolated plating metallayer piece. Each of the isolated plating metal layer pieces preferablyhas a substantially circular shape with a projection area diameter of0.1 μm to 10 μm.

As schematically illustrated by the circulated enlarged view in FIG. 2,the isolated plating metal layer pieces 69 of the light-receivingsurface are formed to be unevenly distributed into the form of bandsparallel to the extending direction of the light-receiving-side fingerelectrodes 60, so that isolated plating metal crowded regions 690 areformed. Each of the isolated plating metal crowded regions is a regionin which the area density of the isolated plating metal layer pieces ishigher than the average area density of the entire isolated platingmetal layer pieces in the entire region where the light-receiving-sidefinger electrodes are not formed. In the isolated plating metal layerpieces crowded regions, the isolated plating metal layer pieces areformed at an area density two or more times more than the average areadensity in the entire region of the light-receiving surface. The areadensity of the isolated plating metal layer pieces is obtained bydividing the light-receiving surface into regions which are each aregion 10 μm square (100 μm²), and calculating out the area of theisolated plating metal layer pieces in each of these regions.

The width of each of the isolated plating metal crowded regions 690 ispreferably 300 μm or less, more preferably 200 μm or less. The lowerlimit of the width of the isolated plating metal crowded region 690 isnot particularly limited. The isolated plating metal layer pieces may bearranged in one straight line. The isolated plating metal crowded regionneed not to be linked with each other over the whole of the region alongthe light-receiving-side finger electrode 60 extends. Thus, the isolatedplating metal crowded regions may partially have one or moreinterruptions. The isolated plating metal crowded regions do not need tobe present in completely parallel to the light-receiving-side fingerelectrodes 60. Thus, the solar cell may have sites where the regions aremeandering to the light-receiving-side finger electrode-extendingdirection.

When the isolated plating metal layer pieces 69 are located to beunevenly distributed in regions in the form of bands parallel to thelight-receiving-side finger electrode 60-extending direction, thecurrent density of the solar cell tends to be improved. Although theisolated plating metal layer pieces deposited on the light-receivingsurface enlarge the light-shielding area, current density increases. Thereason for this is considered to be that sunlight (parallel light rays)incident to the light-receiving-side finger electrodes is reflected onthe isolated plating metal layer pieces so that the quantity of lightentering into the photoelectric conversion section is increased. Lightincident to the finger electrodes is reflected into a direction parallelto the light-receiving surface, or is regularly reflected in the lightincident direction. The light reflected into the light incidentdirection may be again reflected on the interface between a surfaceprotecting layer (e.g., a glass plate) to enter into the photoelectricconversion section, whereas reflected light in the direction parallel tothe light-receiving surface, in general, hardly enters into thephotoelectric conversion section. On the other hand, when the isolatedplating metal layer pieces 69 are formed near the light-receiving-sidefinger electrodes 60, the light reflected, on the finger electrodes,into the direction parallel to the light-receiving surface may bereflected on the isolated plating metal layer pieces. Therefore, amountof light taken into the photoelectric conversion section 50 would beincreased.

It is preferred the isolated plating metal crowded regions 690 of thelight-receiving surface are present 20 μm or more away from the edge ofeach of the light-receiving-side finger electrodes. If the distancebetween the isolated plating metal layer piece 69 and thelight-receiving-side finger electrodes 60 is too small, such aproportion that light scattered and reflected on the isolated platingmetal layer piece again reaches the light-receiving-side fingerelectrode becomes large, and therefore the quantity of reflected lightthat can be taken into the photoelectric conversion section 50 tends todecrease. On the other hand, if the distance between the isolatedplating metal layer pieces 69 and the light-receiving-side fingerelectrodes 60 is too large, such a proportion that light reflected onthe light-receiving-side finger electrode reaches the isolated platingmetal layer piece becomes small, and therefore the proportion ofreflected light which is to be reflected into the atmosphere tends toincrease. For this reason, the distance (separation distance) betweenthe light-receiving-side finger electrodes 60 and the isolated platingmetal crowded regions 690 is preferably 200 μm or less. The distancebetween the light-receiving-side finger electrode 60 and the isolatedplating metal crowded region 690 is more preferably from 30 μm to 150μm, further preferably from 40 μm to 100 μm.

As schematically illustrated in the circled enlarged view in FIG. 3,isolated plating metal layer pieces 79 may be formed also on the surfaceof the second insulating layer 82 on the back side. Almost all of lightthat enters from the back side of the solar cell is near infrared raystransmitted to the back side without being absorbed in the photoelectricconversion section, and re-incident rays obtained in such a manner thatlight incident to gaps between solar cells arranged adjacently isreflected; and thus is non-parallel rays. Therefore, re-incidence effectby scattering and reflection like on the light-receiving side cannot beexpected on the back side, even when the isolated plating metal layerpieces are formed. In the meantime, light incident to the isolatedplating metal layer pieces may cause shadowing loss. Accordingly, anincrease in the area where the isolated plating metal layer pieces aredeposited on the second insulating layer 82 on the back side may reducethe reflected light capturing efficiency from the back surface. It istherefore preferred that the area density of isolated plating metallayer pieces 69 on the surface of the first insulating layer 81 on thelight-receiving side is larger the area density of isolated platingmetal layer pieces 79 on the surface of the second insulating layer 82on the back side. The area density of the isolated plating metal layerpieces on the surface of the first insulating layer on thelight-receiving side is preferably 1.2 times or more, more preferably1.5 times or more, further preferably 2 times or more than the areadensity of the isolated plating metal layer pieces on the surface of thesecond insulating layer. With respect to the area density of theisolated plating metal layer pieces in a range 250 μm or less apart fromthe edge of each of the finger electrodes, the area density on thelight-receiving side is preferably 2 times or more, more preferably 5times or more, further preferably 8 times or more than the area densityof the isolated plating metal layer pieces on the back side.

The method for forming the isolated plating metal layer pieces is notparticularly limited. The pieces are preferably formed by a platingmethod at the same time of forming the plating metal layer 62 of thefinger electrodes 60. It is therefore preferred that the material of theisolated plating metal layer pieces 69 is identical to the material ofthe plating metal layer 62. When the plating metal layer 62 containscopper, it is preferred that the isolated plating metal layer pieces 69also contain copper.

After making openings 89 in the insulating layer 81 on the metal seedlayer 61-non-formed regions, the plating metal layer 62 and the isolatedplating metal layer pieces 69 can be simultaneously formed by depositingplating metal on the transparent electrode layer 41 exposed under theopenings as an origination point of plating. By forming the openings 89along a direction parallel to the metal seed layer 61-extendingdirection, the isolated plating metal layer pieces 69 can be formed,which are unevenly distributed in the form of band-shape extendingparallel to the finger electrode 60-extending direction.

The method for making the openings 89 in the insulating layer 81 on themetal seed layer-non-formed regions is not particularly limited. Theopenings 89 can be made by, e.g., laser scribing, or a mechanical methodsuch as mechanical scribing. The openings 89 can be made into the formof pinholes in the insulating layer 81 by bringing a brush made of resininto contact with the surface of the photoelectric conversion section 50(transparent electrode layer 41) to generate fine particles beforeforming the insulating layer 81, then forming the insulating layer 81thereon. Methods of pressing a porous resin sheet or roller or blowinggrains on the surface of the photoelectric conversion section may alsogenerate particles on the surface of the photoelectric conversionsection so that the openings 89 can be formed in the insulating layer81.

In a preferred embodiment, the openings 89 are formed in the insulatinglayer 81 on the metal seed layer-non-formed regions without scratchingthe surface of the photoelectric conversion section. The openings 89 canbe formed by using the ooze of solvent in the electroconductive pasteused in formation of the metal seed layer 61. In general, the thixotropyof an electroconductive paste is designed in such a manner thatelectroconductive fine particles and binder resin do not ooze out fromthe printed regions even when a large printing pressure is applied.However, as the printing pressure is increased, the ooze amount of thesolvent tends to increase. Solvent oozed out from the electroconductivepaste-formed regions (printed regions) volatilizes when heated insolidifying the paste. When an insulating layer is formed on the regionwhere the solvent has been oozed, pinholes tend to be easily made nearouter peripheral of the solvent oozing regions. By depositing a platingmetal, using the pinholes 89 as origination points, the isolated platingmetal crowded regions 690 can be formed in band-shape extending parallelto the light-receiving-side finger electrode 60-extending direction.

In order to form the isolated plating metal layer pieces 69 on the onthe light-receiving side in a band-shape parallel to thelight-receiving-side finger electrode 60-extending direction and forsuppressing the formation of isolated plating metal layer pieces on theback side, it is sufficient that the printing pressure to theelectroconductive paste is made larger when the first metal seed layer61 is formed on the light-receiving surface, than that to theelectroconductive paste when the second metal seed layer 71 is formed onthe back surface.

By increasing the printing pressure to the electroconductive paste onthe light-receiving side, the solvent in the paste is oozed out to makepinholes in the first insulating layer 81. By reducing the printingpressure to the electroconductive paste on the back side, the ooze ofthe solvent is restrained to suppress generation of pinholes in thesecond insulating layer 82. By this way, the isolated plating metallayer pieces 69 can be selectively formed on the light-receiving side,even when the first plating metal layer 62 on the light-receivingsurface and the second plating metal layer 72 on the back surface aredeposited under the same plating conditions.

As described above, in a case where an interval of thelight-receiving-side finger electrodes 60 is larger than that of theback-side finger electrodes 70 and thus the finger electrodes on theback side are formed more densely, a screen plate having larger aperturearea ratio may be used for printing an electroconductive paste on theback side. Accordingly, in the case of forming the first metal seedlayer on the light-receiving surface and the second metal seed layer onthe back surface under the same printing conditions, the printingpressure on the light-receiving side becomes relatively larger than thatof on the back side.

[Examples of Application to Solar Cells Other than Heterojunction SolarCell]

In the above, the structure of the solar cell of the present inventionwas described with the use of mainly the example of the heterojunctionsolar cell in which the photoelectric conversion section 50 includesconductive silicon-based thin-films 31 and 32 and transparent electrodelayers 41 and 42 on both the surfaces of the conductivesingle-crystalline silicon substrate 10. The invention is applicable tosolar cells other than heterojunction solar cells. Specific examples ofthe other solar cells include a crystalline silicon solar cells otherthe heterojunction type, a solar cell including a semiconductorsubstrate other silicon such as GaAs, a silicon-based thin-film solarcell in which a transparent electrode layer is formed on a pin junctionor pn junction of an amorphous silicon-based thin-film or crystallinesilicon silicon-based thin-film, compound semiconductor solar cell suchas CIS or CIGS, an organic thin-film solar cell such as dye sensitizingsolar cell and one using an organic thin-film (electroconductivepolymer).

[Solar Cell Module]

The solar cells of the present invention are preferably modularized whenput into practical use. In order to modularize the solar cells, anappropriate method is used. For example, as illustrated in FIG. 4,respective bus bar electrodes 66 and 76 of solar cells 100 are connectedto each other through wiring members 105 to form a solar cell string inwhich a plurality of the solar cells are connected to each other inseries or in parallel. The solar cell string is encapsulated withencapsulants 111 and 112, and protecting members 120 and 130 tomodularize the solar cells. The solar cells can be electricallyconnected to the wiring members by, for example, solder connection usinga low-melting-point solder, or connection based on compression using aconductive film (CF).

Transparent resins are suitable for the encapsulant. Examples thereofinclude ethylene/vinyl acetate copolymer (EVA), ethylene/vinylacetate/triallyl isocyanurate (EVAT), polyvinyl butyrate (PVB),silicone, urethane, acrylic, epoxy and olefin resins. In order todecrease costs, the use of EVA is preferred for the light-receiving sideencapsulant 111. When the back-side protecting member 130 containing nometal foil is used, it is preferred to use a polyolefin-containing backside encapsulant as the member 112 for improving reliability of module.Since polyolefin resin has a low water permeability, water can berestrained from invading the photoelectric conversion sections even whenthe back-side protecting member containing no metal foil is used.

It is preferred that the refractive index n₁ of the back sideencapsulant, the refractive index n₂ of the second insulating layer andthe refractive index n₃ of the outermost layer on the back side of thephotoelectric conversion section 50 satisfy the relation: n₁<n₂<n₃. Whenthe refractive index increases step by step from the back side towardthe photoelectric conversion section, reflected light on the back sideis taken into the photoelectric conversion sections even more, so thatthe module conversion efficiency can be enhanced. In the above-mentionedheterojunction solar cell, the outermost layer of the photoelectricconversion section 50 on the back side is the second transparentelectrode layer 42. Therefore, it is sufficient that silicon oxide orany other material having lower refractive index than the transparentelectrode is used for the second insulating layer, and a material havingeven lower refractive index is used for the back side encapsulant. Sincethe refractive index of the encapsulant is generally about 1.5, and thatof the transparent electrode is about 1.9 to 2.3, the insulating layerpreferably has a refractive index in a range from 1.5 to 2.3. Therefractive index is a value at a wavelength of 600 nm, and is measuredby using an ellipsometer.

Examples of the material used for the light-receiving-side protectingmember 120 include, for example, a glass plate (blue glass plate orwhite glass plate), a fluororesin film such as a polyvinyl fluoride film(for example, a TEDLAR film (registered trademark), or a polyethyleneterephthalate (PET) film. From the viewpoint of mechanical strength,optical transmittance, water blocking performance, costs and others, aglass plate is preferred and a white glass plate is particularlypreferred.

The back-side protecting member 130 may be a glass plate, a resin film,a metal foil piece made of aluminum, or a stack body of two or more ofthese members. Since the solar cell of the present invention has apatterned collecting electrode on the back side, the back-sideprotecting member 130 with optical transparency enables the cell tocapture light also from the back side. Therefore, in a solar cell moduleof an installation type, in which light enters from the back side, suchas a flat roof type or on-ground installation type, it is preferred touse the back-side protecting member 130 having optical transparency andcontaining no metal foil.

When the back-side protecting member contains no metal foil, theinvasion of water from the back side tends to increase. In the solarcell of the present invention, both the front and back surfaces of thephotoelectric conversion section are covered with insulating layers.Therefore, water invasion into the photoelectric conversion sections canbe prevented even when the back-side protecting member contains no metalfoil.

In one embodiment, a layer stack in which a black resin layer and aninfrared reflection layer are stacked from the solar cell side is usedas the back-side protecting member 130. The black resin layer hasvisible ray absorbance and absorbs visible rays having wavelengths of800 nm or less. The visible ray transmittance of the black resin layeris preferably 10% or less. The use of the back-side protecting memberincluding the black resin layer gives a solar cell module in which a gapbetween adjacent two solar cells is nonconscious to show a high designproperty since the solar cells are close in external appearance color tothe back-side protecting member.

By arranging the infrared reflection layer on the back side of the blackresin layer, it is possible to reflect light incident to the gap betweenadjacent two solar cells, and near infrared ray transmitted through theback side without being absorbed in the photoelectric conversionsection. The reflected light can be re-entered into the solar cell, sothat the module conversion efficiency can be improved. In the infraredreflection layer, the reflectivity of near infrared rays havingwavelengths of 800 nm to 1200 nm is preferably 80% or more, morepreferably 85% or more, even more preferably 90% or more. In order toradiate the near infrared rays reflected on the infrared reflectionlayer again into the solar cell, the transmittance of infrared rayshaving wavelengths of 800 nm to 1200 nm in the black resin layer ispreferably 80% or more.

For the black resin layer, a resin composition is preferably used whichcontains a colorant such as a pigment or dye, and a thermoplastic resinsuch as polyolefin resin, polyester resin, acrylic resin, fluororesin,or ethylene/vinyl acetate resin. The coloring agent is preferably amaterial absorbing visible rays and transmitting near infrared rays andmay be, for example, a combination of three or more colorants which aredifferent from one another in color hue and each have a lightness L* of45 or more, or a dark-color organic pigment. The black resin layer maycontain an inorganic pigment having an infrared reflective property.

The infrared reflection layer may be, for example, a resin layer made ofa resin composition containing titanium oxide or any other white pigmenthaving an infrared reflective property, or an infrared reflective metalfoil (for example, an aluminum or silver). The metal foil may becorroded or cause short-circuited by exposing to the air. It istherefore preferred to use a resin layer containing no metal foil as theinfrared reflection layer, from the viewpoint of improving reliabilityand safety of the module. For bonding the black resin layer and theinfrared reflection layer, an adhesive layer may be interposedtherebetween.

EXAMPLES

Hereinafter, the present invention will be described in detail byshowing examples, but the present invention is not limited to thefollowing examples.

Example 1

(Texture Formation on Silicon Substrate Surface)

An n-type single-crystalline silicon substrate having a light receivingsurface with a (100) plane orientation and having a thickness of 200 μmwas washed in acetone, immersed in a 2 wt % HF aqueous solution for 5minutes to remove a silicon oxide layer on a surface, and rinsed twicewith ultra-pure water. Washed silicon substrate was immersed for 15minutes in a 5/15 wt % KOH/isopropyl alcohol aqueous solution held at75° C. to perform anisotropic etching. Thereafter, the substrate wasimmersed in a 2 wt % HF aqueous solution for 5 minutes, rinsed twicewith ultra-pure water, and then dried at ambient temperature. Surfacesof the silicon substrate were observed with an AFM to confirm thatquadrangular pyramid-like textured structures having an exposed (111)surface were formed on each of the front and back surfaces Thearithmetic mean roughness thereof was 2100 nm.

(Formation of Silicon-Based Thin-Films)

The texture-formed single-crystalline silicon substrate was introducedinto a CVD apparatus, on the light-receiving surface thereof, anintrinsic amorphous silicon layer was deposited to have a thickness of 4nm and a p-type amorphous silicon layer was deposited thereon to have athickness of 5 nm. The thickness of the thin-film in this example is avalue calculated from a deposition rate determined by measuring thethickness of a thin-film formed on a glass substrate under the sameconditions using a spectroscopic ellipsometer (trade name: M2000,manufactured by J.A. Woollam Co. Inc.).

Deposition conditions of the intrinsic amorphous silicon layer includeda substrate temperature of 180° C., a pressure of 130 Pa, a SiH₄/H₂ flowratio of 2/10 and a supplied power density of 0.03 W/cm². Depositionconditions of the p-type amorphous silicon layer included a substratetemperature of 190° C., a pressure of 130 Pa, a SiH₄/H₂/B₂H₆ flow ratioof 1/10/3 and a supplied power density of 0.04 W/cm². With respect tothe B₂H₆ gas mentioned above, a diluting gas wherein B₂H₆ was dilutedwith H₂ gas to have a concentration of 5000 ppm was used.

Thereafter, on the back surface of the silicon substrate an intrinsicamorphous silicon layer was deposited to have a thickness of 5 nm. Onthe intrinsic amorphous silicon layer, an n-type amorphous silicon layerwas deposited to have a thickness of 10 nm. Deposition conditions of then-type amorphous silicon layer included a substrate temperature of 180°C., a pressure of 60 Pa, a SiH₄/PH₃ flow ratio of 1/2 and a suppliedpower density of 0.02 W/cm². With respect to the PH₃ gas mentionedabove, a diluting gas wherein PH₃ was diluted with H₂ gas to have aconcentration of 5000 ppm was used.

(Deposition of Transparent Electrode Layers)

The substrate on which the silicon-based thin-films were formed wastransferred to an RPD apparatus, and an 80 nm-thick indium oxide layerwas formed as a transparent electrode layer on each of the p-typeamorphous silicon layer and the n-type amorphous silicon layer. 1%tungsten doped In₂O₃ was used as a vapor deposition source. Indeposition of the transparent electrode layer on the light-receivingside (on the p-type amorphous silicon layer), a peripheral region 0.5 to0.75 mm from the edge was covered with a mask to avoid deposition ofindium oxide layer on the peripheral portion. In deposition of thetransparent electrode layer on the back side (on the n-type amorphoussilicon layer), no mask was used to deposit the indium oxide layer onthe whole of the surface.

(Formation of Metal Seed Layers)

An electroconductive paste was screen-printed on the indium oxide layeron the light-receiving side to form metal seeds. A paste used forformation of the metal seeds on the light-receiving side included: SnBimetal powder (particle size D_(L)=25 to 35 μm; and melting point T₁=141°C.) and silver powder (particle size D_(H)=2 to 3 μm; and melting pointT₂=971° C.) at a ratio by mass of 20/80 as electroconductive fineparticles; an epoxy resin (5 wt %) as a binder resin; and a solvent.This electroconductive paste was screen-printed on the substrate, usinga screen plate having openings corresponding to a pattern of bus barelectrodes and finger electrodes (finger electrode width: 70 μm; andfinger electrode pitch: 2 mm), and then the resultant was temporarilybaked at 140° C. for about 20 minutes.

Next, in the same manner as on the light-receiving side, theelectroconductive paste was printed on the indium oxide layer on theback side, and the resultant was temporarily baked to form metal seeds.For formation of the metal seeds on the back side, a screen plate havingprinting openings corresponding to a finger electrode width of 60 μm anda finger electrode pitch of 0.75 mm was used.

The surfaces of the substrate on which the metal seeds were formed wereobserved with an optical microscope. On the light-receiving side,volatilization traces of the solvent in the electroconductive paste inthe printing were observed in a region over lengths of 50 to 200 μm fromthe end of the metal seeds. On the back side, no volatilization traceswere observed.

(Formation of Insulating Layers)

The substrate was transferred to a CVD apparatus, and a 40 nm-thicksilicon oxide layer was deposited on the light-receiving surface.Thereafter, the substrate was turned upside down, and a 60 nm-thicksilicon oxide layer was formed on the back surface. Depositionconditions of the silicon oxide included a substrate temperature of 180°C., a pressure of 60 Pa, a SiH₄/CO₂ flow ratio of 1/10 and a suppliedpower density of 0.04 W/cm². After formation of the silicon oxide layeron the back side, the insulating layer on the light-receiving side had athickness of 40 nm at a central portion and 60 nm at a peripheralportion. The thickness at the peripheral portion was increased.

Surface shape change due to thermal-fluidization of the metallicmaterial in the metal seeds and degassing from the metal seeds occur bythe heating during formation of the silicon oxide layer, and therebymany pinholes were formed in the silicon oxide layers deposited on themetal seeds. On the light-receiving side, pinholes were also formedlocated peripheral of volatilization traces of the solvent in the paste.

(Formation of Plating Metal Layers)

A probe was connected to the metal seeds in bus bar regions on each ofthe light-receiving side and the back side. The substrate was immersedin a copper plating solution to perform electroplating, so that a copperplating layer was deposited to have a thickness of about 10 μm on themetal seeds. The substrate was rinsed with pure water, and then wasimmersed in a tin plating solution to perform electroplating. A tinplating layer was deposited to have a thickness of about 3 μm on each ofouter surfaces of the copper layers. Thereafter, the substrate wasrinsed with pure water. In this way, on each of the light-receiving sideand the back side, a plating metal layer consisting of a stack of thecopper plating layer and the tin plating layer was formed onto the seedmetal layer.

Surfaces after the electroplating were observed with an opticalmicroscope. FIGS. 5 and 6 show microscope observation images of thelight-receiving surface and back surface, respectively. On thelight-receiving side, in regions 50 to 200 μm apart from the end of themetal seed layer, many isolated plating metal layer pieces each having adiameter of about 1 to 10 μm were deposited to form isolated platingmetal crowded regions. On the back side, isolated plating metal layerpieces were formed at random, but no isolated plating metal crowdedregions were formed.

(Modularization)

Four solar cells obtained above were used to produce a mini-module.Initially, light-receiving side bus bars and the back side bus bars ofadjacent two solar cells were connected via tab connector using a solderto obtain a solar cell string in which the four solar cells wereconnected to one another in series. EVA resin sheets as encapsulantswere arranged on the light-receiving side and the back side of the solarcell string. A strengthened white glass piece as a light-receiving-sideprotecting member and a transparent resin sheet containing atetrafluoroethylene/ethylene copolymer (ETFE) resin as a back-sideprotecting member were arranged thereon. The resultant was subjected tovacuum drawing, and then heated at 150° C. for about 30 minutes to causethe EVA to undergo crosslinking reaction. In this way, the string wasencapsulated.

Comparative Example 1

In the same manner as in Example 1, solar cells and a mini-module wereproduced except that, in formation of the light-receiving side metalseed layer, the printing pressure for the screen printing was made smallto restrain the solvent in the electroconductive paste from oozing.After formation of the metal seed layers, surfaces of the substrate wereobserved with an optical microscope to confirm that no volatilizationtraces were formed on each of the light-receiving side and the backside. The surfaces after the electroplating were observed with anoptical microscope to confirm, on each of the light-receiving side andthe back side, formation of isolated plating metal layer pieces atrandom but isolated plating metal crowded regions were not formed.

Comparative Example 2

In the same manner as in Example 1, solar cells and a mini-module wereproduced except the followings: the printing pressure for the screenprinting was made small to restrain the solvent in the electroconductivepaste from oozing in formation of the light-receiving side metal seedlayer; and the printing pressure for the screen printing was made largeto ooze the solvent in the electroconductive paste in formation of theback side metal seed layer. After formation of the metal seed layers,surfaces of the substrate were observed with an optical microscope toconfirm that no volatilization traces were formed on the light-receivingside, and that vaporization traces of the solvent were formed in regionsover lengths of 50 to 300 μm from ends of the metal seeds. The surfacesafter the electroplating were observed with an optical microscope. Onthe light-receiving side, isolated plating metal layer pieces wereformed at random, but no isolated plating metal crowded regions wereformed. On the back side, in regions 50 to 300 μm apart from ends of themetal seeds, many isolated plating metal layer pieces each having adiameter of about 1 to 10 μm were deposited to form isolated platingmetal crowded regions.

[Evaluation]

I-V measurement of the mini-module obtained in each of the Example andthe Comparative Examples was carried out with using a solar simulatorhaving a light reflective metal on the back side. Short circuit currentdensity (Jsc), open circuit voltage (Voc), fill factor (FF) andconversion efficiency (Effl are shown in Table 1. In Table 1, each ofthe conversion characteristics are shown as a relative value with thevalues of the mini-module of Comparative Example 1 as a reference value(regarded as one).

TABLE 1 Jsc Voc FF Eff Example 1 1.005 1.000 1.000 1.005 ComparativeExample 1 1 1 1 1 Comparative Example 2 0.998 1.000 1.000 0.998

In Comparative Example 2, in which the isolated plating metal layerpieces were disposed near the finger electrodes on the back side, Jscwas lower than in Comparative Example 1, which had no isolated platingmetal crowded regions. It can be considered that this result was causedby the fact that Comparative Example 2 was increased in light shieldingarea by the isolated plating metal layer pieces on the back side, sothat amount of captured light from the back side was decreased. In themeantime, in Example 1, in which the isolated plating metal layer pieceswere disposed which were crowded in the band shape area near the fingerelectrodes on the light-receiving side, Jsc became larger than inComparative Example 1 although Example 1 was large in metal layer-formedarea on the light-receiving surface. It can be considered that thisresult was caused by the fact that sunlight (parallel rays) wasreflected on the finger electrodes on the light-receiving side, and thenreflected on the isolated plating metal layer pieces to increase theamount of light entering into the photoelectric conversion section.

DESCRIPTION OF REFERENCE CHARACTERS

-   -   10: silicon substrate    -   21, 22: intrinsic silicon-based thin-film    -   31, 32: conductive silicon-based thin-film    -   41, 42: transparent electrode layer    -   50: photoelectric conversion section    -   81, 82: insulating layer    -   60, 70: finger electrode    -   61, 71: metal seed layer    -   62, 72: plating metal layer    -   69: isolated plating metal layer piece    -   690: isolated plating metal crowded region    -   100: solar cell    -   105: wiring member    -   111, 112: encapsulant    -   120, 130: protecting member

The invention claimed is:
 1. A solar cell comprising: a photoelectricconversion section including a semiconductor junction having alight-receiving surface and a back surface opposite the light-receivingsurface; a plurality of light-receiving-side finger electrodes on thelight-receiving surface of the photoelectric conversion section,extending in a direction; a plurality of back-side finger electrodes onthe back surface of the photoelectric conversion section; a firstinsulating layer over the light-receiving surface of the photoelectricconversion section; and a plurality of first isolated plating metallayer pieces over the first insulating layer, the plurality of firstisolated plating metal layer pieces each having a substantially circularshape, each having a diameter less than a width of each of thelight-receiving-side finger electrodes and being isolated from thelight-receiving-side finger electrodes and the back-side fingerelectrodes on the first insulating layer, wherein thelight-receiving-side finger electrodes of the plurality oflight-receiving-side finger electrodes each include: a first metal seedlayer between the photoelectric conversion section and the firstinsulating layer; and a first plating metal layer over the firstinsulating layer, the first plating metal layer being coupled with thefirst metal seed layer by way of openings in the first insulating layer,the plurality of first isolated plating metal layer pieces aredistributed in a first isolated plating metal crowded region over thefirst insulating layer having an area density of the plurality of firstisolated plating metal layer pieces included in the first isolatedplating metal crowded region that is two times or more than an averagearea density of the plurality of first isolated plating metal layerpieces over an entirety of a region over the first insulating layer freefrom having the light-receiving-side finger electrodes, the firstisolated plating metal crowded region includes 20 or more of the firstisolated plating metal layer pieces distributed over the entire firstisolated plating metal crowded region, the first isolated plating metalcrowded region extends along the light-receiving-side finger electrodesand is located 20 μm or more and 200 μm or less away from thelight-receiving-side finger electrodes in a direction perpendicular tothe extending direction of the light-receiving-side finger electrodes,and the first metal seed layer is not provided under the first isolatedplating metal crowded region.
 2. The solar cell according to claim 1,further comprising: a second insulating layer on the back surface of thephotoelectric conversion section; and a plurality of second isolatedplating metal layer pieces, the second insulating layer being betweenthe photoelectric conversion section and the second isolated metal layerpieces, the second isolated plating metal layer pieces being isolatedfrom the light-receiving-side finger electrodes and the back-side fingerelectrodes on the second insulating layer, wherein the back-side fingerelectrodes of the plurality of back-side finger electrodes each include:a second metal seed layer between the photoelectric conversion sectionand the second insulating layer; and a second plating metal layer, thesecond insulating layer being between the second plating metal layer andthe second metal seed layer, and the second plating metal layer beingcoupled with the second metal seed layer by way of openings in thesecond insulating layer, and the area density of the first isolatedplating metal layer pieces over the first insulating layer is greaterthan an area density of the second isolated plating metal layer piecesover the second insulating layer.
 3. The solar cell according to claim2, wherein each of the first insulating layer and the second insulatinglayer is an inorganic layer having a thickness of 10 to 200 nm.
 4. Thesolar cell according to claim 2, wherein the first plating metal layer,the second plating metal layer and the first isolated plating metallayer pieces each contain copper.
 5. The solar cell according to claim1, wherein a separation distance between two adjacentlight-receiving-side finger electrodes is larger than a separationdistance between two adjacent back-side finger electrodes.
 6. The solarcell according to claim 1, wherein the photoelectric conversion sectionincludes: a conductive single-crystalline silicon substrate having afirst surface and a second surface opposite the first surface; a firstsilicon based thin-film having a first conductivity-type over the firstsurface of the conductive single-crystalline silicon substrate; a firsttransparent electrode layer over the first surface of the conductivesingle-crystalline silicon substrate; a second silicon based thin-filmhaving a second conductivity-type on a second surface side of theconductive single-crystalline silicon substrate; and a secondtransparent electrode layer on the second surface side of the conductivesingle-crystalline silicon substrate.
 7. The solar cell according toclaim 1, wherein the first isolated plating metal crowded region has aband-shape.
 8. The solar cell according to claim 1, wherein the diameterof each first isolated plating metal layer piece is 0.1 μm to 10 μm. 9.A solar cell module comprising: a solar cell according to claim 1, thesolar cell having a light-receiving side and a back side, alight-receiving-side protecting member on the light-receiving side ofthe solar cell; a first encapsulant between the light-receiving side ofthe solar cell and the light-receiving-side protecting member; aback-side protecting member on the back side of the solar cell; and asecond encapsulant between the back side of the solar cell and theback-side protecting member.
 10. The solar cell module according toclaim 9, wherein the light-receiving-side protecting member comprisesglass, and the second encapsulant includes a polyolefin resin.
 11. Thesolar cell module according to claim 9, wherein the back-side protectingmember includes no metal foil.
 12. The solar cell module according toclaim 9, wherein the solar cell comprises a second insulating layer onthe back surface of the photoelectric conversion section, and refractiveindices n₁, n₂ and n₃ satisfy a relation: n₁<n₂<n₃, where n₁ is arefractive index of the second encapsulant, n₂ is a refractive index ofthe second insulating layer, and n₃ is a refractive index of anoutermost layer on the back side of the photoelectric conversionsection.
 13. The solar cell module according to claim 9, wherein theback-side protecting member includes a black resin layer having visibleray absorbance and an infrared reflection layer on the black resin layerstacked in this order on the back side of the solar cell.